Eventually, ROMY scientists will monitor changes in the length of the day and the
position of the poles. Neither is as fixed as
you might think, varying by milliseconds
and centimeters each day. The sun and moon
tug on the planet, while the drift of continents, changes in ocean currents, and the
rebounding of the crust since the retreat of
ice age glaciers all shift mass around, altering Earth’s moment of inertia and therefore
its spin. Even hurricanes and earthquakes
can give a tiny nudge this way or that.

Earth’s little twitches have practical consequences. Precisely targeting a rocket, whether
it is destined for Mars or geostationary orbit,
requires taking them into account. And the
data from GPS satellites—which businesses
and consumers the world over use—would
drift into irrelevance within weeks if their
exact positions in relation to Earth’s surface
were not constantly corrected.

Currently, the best measurements of those
variables come from a system called very-long-baseline interferometry (VLBI), which
uses radio dishes spaced
across Earth to stare at quasars—brilliant beacons in the
distant universe that occasionally flicker. By clocking
when widely spaced dishes
record a change in brightness,
geodesists can calculate the
planet’s spin rate and its axis.

But the system requires dozens of observatories to give up
valuable astronomy time, and
for the best timing comparisons, hard drives have to be shipped overnight from remote locales to supercomputer
centers. It can take days to turn observations
into a published measurement.

ROMY will try to match the precision
of VLBI—and outdo it in speed. In theory,
ROMY could monitor Earth’s spin rate and
axis constantly, updating measurements in
real time, says Lucia Plank, a geodesist at the
University of Tasmania in Hobart, Australia,
who helps provide the VLBI service. “The
advantage of ROMY is you have an instantaneous result,” Plank says, though she adds
that the VLBI technique, being more stable,
is unlikely to go away anytime soon.

Whereas VLBI measures Earth’s rotation
with respect to markers billions of light-years away, ROMY measures it right at the
surface—and the difference could be telling.
That’s because Einstein’s frame-dragging
effect, in which the gravity of Earth’s rotating mass warps and twists nearby spacetime,
should cause an infinitesimal shift in the
rotation rate as measured close to Earth. It’s
the same test that was done, famously and
expensively, by Gravity Probe B, a $750 million

NASA mission that put gyroscopes on a satellite and measured the frame-dragging. Belfi
says that doing it again, from the ground, is
worthwhile. “In physics this is not a trivial
result,” says Belfi, who wants to use GINGER
to do the test if ROMY cannot.

BEING SO NEW, ROMY is plagued by experimental drift. The structure is still settling
in the soft sediments of Fürstenfeldbruck.
Unlike other ring lasers, which were fixed
to blocks of Zerodur—a ceramic resistant
to temperature changes—ROMY’s steel
tubes flex with the temperature swings
of day and night. It also is prone to shifting after rains saturate the ground. Igel
eventually wants to eliminate those drifts
by putting small motors behind each of
ROMY’s mirrors to make tiny adjustments
to the rings in real time. But he is keen to
embrace one type of fast-moving “drift”:
earthquake shaking.

In the past, seismologists have measuredonly translation—the displacement ofthe ground along any of the three cardinalaxes. But seismic waves alsodrive tilt motions, whichrotate points without shift-ing their positions. Tradi-tional seismometers couldnot measure tilt motions,but theory suggested, reas-suringly, that they are smallenough to ignore. As CharlesRichter, the seismologistwho developed the famousmagnitude scale for earth-quakes, wrote in 1958, “suchrotations are negligible.”“But they are there,” Igel says. Indeed,experiments in recent years have sug-gested that the motions can actually belarge. Soft soils can amplify them to 10%or more of the magnitude of translationalmotions. Engineers have been designingbuildings only for translational shaking,but they should take tilts into account aswell, says John Evans, a seismologist withthe U.S. Geological Survey in Santa Cruz,California. “It’s best to know what [shak-ing] actually goes into a building to makeits response within tolerable limits.”Measurements of tilt also could paydividends for earth science. Traditionalseismometers can misclassify tilting astranslational motion—a problem especiallyacute for ocean bottom sensors that sit onsoft muds, Evans says. By measuring tiltdirectly, researchers could limit such “datacontamination.” Tilt measurements alsomight sharpen 3D models of the interiorsof volcanoes, where swelling magmas cre-ate tremors with larger-than-normal rota-tions, Igel says. “If you do not take intoaccount these tilt motions, your modelmight be wrong,” he says.

ROMY should help earth scientists
explore this new seismological frontier—if
only by showing that it exists. Soon after
the team turned on its first triangular ring,
it sensed rotations from the magnitude-6.6
Norcia earthquake in Italy last October.

EVENTUALLY, scientists will want to get
closer to the source. “You cannot move
ROMY,” says Frédéric Guattari, head of seismic rotation sensors at iXBlue, a navigation sensor company in Paris. “Now, we
need a portable device.” The answer from
iXBlue is a compact sensor that relies not on
lasers but on a fiber optic loop 5 kilometers
long, wound into a coil just 20 centimeters
across. The device sends photons in opposite directions through the loop, interferes
them, and tracks phase shifts to detect
rotations. Guattari has already placed
prototypes astride the Stromboli volcano
and in the Florence cathedral.

At up to €50,000 each, the sensors will
be much more expensive than a traditional
seismometer, but Guattari says they will
ultimately offer a cheaper way to map the
subsurface. Typically, geoscientists search
for oil and gas traps deep in Earth by laying out dozens or even hundreds of sensors
in an array. The array listens for the echoes
of seismic waves—generated by distant
earthquakes or small explosions detonated
nearby—as they bounce off subsurface
structure. But by measuring rotation as well
as translation, seismologists can get not
only the displacement of earthquake waves
but also their velocities, which are a powerful probe of subsurface structure. “You
can do a lot more with this point measurement,” Igel says.

Technology from iXBlue might allow theoil and gas industry to get by with fewersensors. It also could prove useful in situ-ations when deploying even one sensor ischallenging—such as on missions to otherplanets. Evans predicts that tilt sensorscould flourish. “I think we’re going to seeslow adoption,” he says. “In 20 years theycould be standard.”But Igel and Schreiber hope that it won’tbe just the small fry that proliferate—theyalso want ROMY to spawn offspring. Withmultiple large ring lasers scattered aroundthe globe, geodetic measurements couldbe coordinated, calibrated, and checkedagainst one another to create a richer andmore precise picture of our planet’s twistsand turns. Plank, though loyal to VLBI,says she shares the hope that Germany’sgreat ring won’t reign alone. “The ulti-mate goal would be to have more of thesearound the globe.” j